American Black Hawk helicopters and at least eight U.S. armored vehicles rushed to support dozens of Afghan troops battling the assailants at the three-building police complex in restive Kandahar province. It was unclear how many insurgents were involved in the attack.

Thursday's attack began when four suicide bombers stormed police complex, said Zalmai Ayubi, a spokesman for the provincial governor. Three of the bombers blew themselves up, Ayubi said, while the fourth man exchanged fire with Afghan troops.

"I heard a blast and after that continuous fighting with rocket launchers," said Kandahar provincial policeman Ashrafullah Agha. After a third large explosion, Agha cut off the interview.

Four intelligence officers were killed in the fighting, along with an Afghan soldier and a police officer, said provincial police chief Khan Mohammad Mujahid. He said 12 police were wounded.

The police complex sits near the main highway between Kandahar and the capital of Kabul, 148 miles to the north. A weapons training school and police literacy center are located inside the complex. Kandahar is a Taliban stronghold that has seen some of the most intense fighting of the war.

Also on Thursday, NATO announced that it had concluded what it described as a "significant operation" in mountainous eastern Kunar province near the border.

More than 80 insurgents were killed after joint Afghan and coalition forces cleared militants out of four villages, NATO said.

"The combined security force moved into the mountainous area near the border in order to disrupt insurgent activities in the region," NATO said in a statement.

"Insurgents had been using the area to move supplies and men into Kunar province and to stage attacks on Afghan and coalition forces."